If it were cnc'd than yes hollow would cost more as it would take more time. Seeing it's built layer by layer, you are only charged for the volume. Now with some materials there are density discounts (volume compared to bounding box). To check you can look down the right side of the item page, and there you'll see the dimensions of the bounding box, and the volume. You can compare the dimensions in your software and the volume if you software has a function for it.

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michael@shapeways.com Community Advocate